There are a variety of instruments used to measure predefined parameters. Many of these instruments are calibrated after assembly and tested for proper calibration by using a test sample which has known parameters. The parameters of the test sample are measured by the instrument. If properly calibrated, the instrument provides an indication of the known parameters. If not properly calibrated, the instrument outputs data different than the known parameters. The measuring instrument can then be manually calibrated to correctly reflect the known parameters.
A testing apparatus which includes a measuring system is disclosed in U.S. Pat. No. 3,187,440 to Merrill et al. entitled "Dynamic Wheel Alignment Testing Apparatus." The wheel aligner described therein automatically checks and measures specified interrelated angles which are pertinent to aligning the front wheel suspension system of a vehicle. Among such angles, toe in is defined as the inward slanting of the wheels toward the front while camber is defined as the inward sloping of the wheels toward the bottom. The caster and kingpin inclinations are mathematically related to the camber angle of the wheel. The measuring system provides a visual indication of each of the relevant inclinations; however, it is assumed that the measuring system is properly calibrated prior to determining the various angles. That is to say, it is expected, if the toe display meter indicates an angle of zero degrees, that there is no inward slanting of the wheels toward the front. However, if signals from the electronics circuitry which control meter movement are altered or have drifted from initial reference levels, the toe display meter may register a value other than zero degrees even though the angle should provide a reading of zero degrees. Such discrepancies between the visual meter indication and the actual angles pertinent to the front wheel suspension system result in the determination of inaccurate wheel alignment data and the making of incorrect wheel alignments.